Glycoprotein pharmaceuticals are major targets of the biotechnology industry and include such widely used therapeutic agents as tissue plasminogen activator (TPA), erythropoietin (EPO) and monoclonal antibodies. Glycosylation presents special challenges in drug discovery and development, largely due to the heterogeneity of oligosaccharide structures obtained from recombinant expression in eukaryotic cell lines. The presence of heterogeneous glycoforms convolutes the characterization of the glycoprotein's structure and biological activity, which hinders clinical evaluation and approval. New strategies for their production which control oligosaccharide structure and uniformity would facilitate the development of glycoprotein pharmaceutical agents.
We have developed novel methods for the synthesis of glycopeptides based on the highly selective reaction of nucleophilic carbohydtrate derivatives with ketone-containing peptides. Peptides bearing unnatural ketone side chains can be generated using N-protectd (2S)-aminolevulinic acid by standard solid-phase peptide synthesis (SPPS). Oligosaccharides functionalized at their reducing termini with aminooxy, hydrazide or thiosemicarbazide groups can be coupled to keto-peptides in aqueous solvent without need for protecting groups or auxiliary coupling reagents. These methods can be used to prepare glycopeptides of therapeutic interest.